Phosphine-promoted cyclization of dicyclopropenones

Article abstract of DOI:10.1002/adsc.201300875

A novel phosphine-promoted intramolecular cyclization of dicyclopropenones 1 has been described in this contribution. A variety of 2,3-dihydro-1H-indene-4, 6-diol derivatives 2 and hexahydropentalen-2-one derivatives 3 were obtained selectively in moderat

Full text of DOI:10.1002/adsc.201300875

COMMUNICATIONS
DOI: 10.1002/adsc.201300875
Phosphine-Promoted Cyclization of Dicyclopropenones
Jin-Ming Yang,a Xiang-Ying Tang,b Yin Wei,b,* and Min Shia,b,*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, 130 Mei Long Road, Shanghai 200237, Peopleꢀs Republic of China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, 354 Fenglin Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: weiyin@mail.sioc.ac.cn or Mshi@mail.sioc.ac.cn
Received: September 29, 2013; Published online: December 5, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300875.
Abstract: A novel phosphine-promoted intramolec-
ular cyclization of dicyclopropenones 1 has been de-
scribed in this contribution. A variety of 2,3-dihy-
dro-1H-indene-4,6-diol derivatives 2 and hexahy-
dropentalen-2-one derivatives 3 were obtained se-
lectively in moderate to good yields under mild
conditions through different phosphine-promoted
reactions.
nones and alkynes to afford cyclopentadienones. And
Lꢁpez-Leonardo et al.[13e] focused on the reaction of
diphenylcyclopropenone with a series of N-alkyl- and
N-arylamino-P,P-diphenylphosphanes to produce the
ketene intermediates and in turn to afford b-
phosphinACTHNUTRGNEUGoN yl carboxamides under the standard condi-
tions.[14] More recently, Lambert and co-workers dis-
closed a catalytic strategy for the promotion of dehy-
drative reactions[15e] using simple cyclopropenones as
catalysts. This strategy, which relies on the facile for-
mation of cyclopropenium cations for substrate acti-
vation,[15] was first demonstrated in the context of al-
cohol chlorodehydration using oxalyl chloride as the
activating agent and source of nucleophile. Inspired
by the chemistry of cyclopropenones and as a part of
Keywords: cyclization; cyclopropenones; phos-
phines; selectivity
Cyclopropenones, first prepared by Breslow et al.[1] our continuing interest on nucleophilic phosphine-
and Volpin and co-workers[2] more than 50 years ago, mediated reactions, we systematically investigated the
have often been developed as useful building blocks reactions of dicyclopropenones in the presence of
for the construction of natural products and bioactive phosphines. Herein, we wish to report a novel phos-
compounds.[3] Cyclopropenones possessing amphiphil- phine-promoted cyclization of dicyclopropenones.
ic properties can act both as good electrophiles for
Initially, we started our investigation by using dicy-
1,2- or 1,4-addition and as precursors for the stable clopropenone 1a, 20 mol% triphenylphosphine and
2p-aromatic hydroxycyclopropenium cation generated 1.0 equiv. of water in dry tetrahydrofuran (THF) at
readily by protonation of the carbonyl group.[3a,4–6] room temperature, the desired product dimethyl 4,6-
The chemical reactivity of the cyclopropenone ring is dihydroxy-5,7-diphenyl-1H-indene-2,2(3H)-dicarboxy-
dominated by two processes:[4,7] (i) nucleophilic attack late 2a was obtained in 16% yield with 47% of the
on the carbonyl carbon with the formation of acrylic starting materials being recovered after 12 h (Table 1,
acid derivatives[8] and (ii) decarbonylation to alkynes entry 1). The structure of 2a has been unequivocally
under high temperature pyrolysis or in the presence confirmed by X-ray diffraction studies.[16] Based on
of various catalysts.[9] Acetylenes were also observed our previous work,[17] we increased the phosphine
as intermediates[10,11] or were even isolated[1e,8,12] in the loading to 100 mol% (Table 1, entry 2), and com-
photolysis of some cyclopropenones. This remarkable pound 2a was obtained in 48% yield. Decreasing the
three-membered small ring has gained much attention phosphine loading to 50 mol% (Table 1, entry 3) gave
in the past decades.[13] Boger and Nakamura have re- a similar result. Other phosphines, such as tris(para-
ported that cyclopropenone ketals, either thermally tolyl)phosphine,
tris(3,5-dimethylphenyl)phosphine,
(via dipolar cycloaddition)[13a] or with a Pd
ACTHNUTRGNEU(NG OAc)2 cat- tris(4-methoxyphenyl)phosphine gave no better re-
alyst,[13b] undergo cycloaddition with various p-sys- sults than PPh3 (Table 1, entries 4–6). To our delight,
tems to provide cyclopentadienones after ketal hy- use of 50 mol% of bis(diphenylphosphino)methane
drolysis. In 2006, Wender[13c] reported rhodium(I)-cat- (dppm) significantly increased the yield of the desired
alyzed [3+2]cycloaddition reactions of cycloprope- product 2a to 79% (Table 1, entry 7). Instead of THF,
Adv. Synth. Catal. 2013, 355, 3545 – 3552
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3545

Jin-Ming Yang et al.
COMMUNICATIONS
Table 1. Optimization of the reaction conditions.[a]
[a]
[b]
[c]
[d]
Reaction conditions: 0.1 mmol 1a, X mol% catalyst, Y equiv. H2O, 1.0 mL dry solvent.
Isolated yields.
47% of 1a were recovered.
The reaction was run at room temperature for 5 h.
use of dichloromethane (DCM) as the solvent slightly propriate substrates, and the cyclization of dicyclopro-
increased the yield of 2a (Table 1, entry 8). Shortening penones proceeded smoothly to give the correspond-
the reaction time to 5 h did not influence the yield ing products 2b–2e in moderate yields. Next, the influ-
(Table 1, entry 9) and increasing the amount of water ence of the terminal aryl moieties was investigated.
decreased the yield (Table 1, entries 10 and 11). Varia- Dicyclopropenones 1f–1l, having electron-donating
tion of the phosphines or solvents (see the Supporting alkyl and methoxy substituents or electron-withdraw-
Information) led to a decrease in chemical yield. In- ing halogen substituents, underwent the reactions
terestingly, when alkylphosphines were employed, the smoothly to give 2f–2l in 58–73% yields. Moreover,
desired product 2a and another new product 3a were increasing the electron density of the aryl functionali-
obtained (Table 1, entries 12–18). The structure of 3a ties in dicyclopropenone 1n by introducing two elec-
has been unequivocally confirmed by X-ray diffrac- tron-donating methyl groups led to the formation of
tion studies.[18] When the reaction was carried out in the corresponding product 2n in 80% yield. Further-
THF with 1.0 equiv. dimethylphenylphosphine, com- more, a range of substrate types including those con-
pounds 2a and 3a were obtained in 20% and 64% taining protected alcohols (1q and 1r) and those teth-
yields after 12 h, respectively (Table 1, entry 19). ered by a nitrogen or an oxygen atom (1u and 1v)
Thus, finally, we established the optimal reaction con- were also suitable for this cyclization reaction, giving
ditions: 50 mol% of dppm in DCM at room tempera- the corresponding cyclized products in moderate
ture for the synthesis of compound 2a or 100 mol% of yields. The simple carbon tether analogue 1p without
dimethylphenylphosphine in THF at room tempera- the gem-disubstituent effect also provided the indene
ture for the synthesis of compound 3a.
derivative 2p in 55% yield. When the substituents on
With these optimized reaction conditions in hand, the aryl moieties were different (1s), the correspond-
we turned our attention to determine the scope and ing products 2s and 2s’ could also be obtained as
limitations of the reaction in the presence of dppm a mixture in 72% yield, with a 1:1.4 ratio. When the
(50 mol%). As summarized in Table 2, diethyl, diiso- substituent on the aryl moieties was trifluoromethyl
propyl, di-tert-butyl and dibenzyl malonates were ap- (1m) and the dicyclopropenone 1o with thiophene ter-
3546
asc.wiley-vch.de
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 3545 – 3552

Phosphine-Promoted Cyclization of Dicyclopropenones
Table 2. Scope of the cyclization in the presence of dppm.[a]
[a]
Reaction conditions: 0.1 mmol 1, 50 mol% dppm, 1.0 equiv. H2O, 1.0 mL DCM, yields are of isolat-
ed products.
The solvent was THF.
[b]
[c]
The reaction was run in THF at room temperature with 1.0 equiv. Me2PhP and 1.0 equiv. H2O for
48 h.
minal groups could also undergo the cyclization, how- the optimized conditions (Table 1, entry 19); the re-
ever, affording the corresponding products in 47% sults are presented in Table 3. As can be seen from
yields. Only when the terminal group is an aliphatic Table 3, all of the reactions proceeded smoothly to
group such as substrates 1t (see the Supporting Infor- give the corresponding products 3b–3e in 39–58%
mation), the reaction did not provide the desired yields at room temperature when the substituents of
product under the standard conditions.
the malonates were diethyl, diisopropyl, di-tert-butyl
Further experiments were performed to extend the and dibenzyl. Dicyclopropenone 1j, having an elec-
scope of this cyclization to afford compounds 3 under tron-withdrawing chlorine substituent on the benzene
Adv. Synth. Catal. 2013, 355, 3545 – 3552
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3547

Jin-Ming Yang et al.
COMMUNICATIONS
Table 3. Scope of the cyclization in the presence of Me2PhP.
[a]
[b]
Reaction conditions: 0.1 mmol 1, 1.0 equiv. Me2PhP, 1.0 equiv. H2O, 1.0 mL THF, yields are of iso-
lated products.
A 20% yield of 2b was obtained.
ring, was smoothly converted to 3f in 48% yield. The This result suggested that the D of 3c-d was trans-
simple carbon tether analogue 1p without the gem- ferred from D2O.
disubstituent effect also provided the desired product
Plausible mechanisms for this reaction are outlined
3g in 51% yield. Moreover, substrates containing pro- in Scheme 2 on the basis of the above isotopic label-
tected alcohols such as 1q and 1r were also suitable ing and control experiments. The transformation is
for this cyclization, providing products 3h and 3i in believed to proceed via conjugate addition of phos-
43% and 45% yields, respectively.
phine to one of the cyclopropenones, affording zwit-
The unique phosphine-promoted cyclization of di- terionic species A, followed by [2+2]addition to form
cyclopropenones encouraged us to investigate the re- intermediate C stepwise via intermediate B. From in-
action mechanism. First, we have done 31P NMR trac- termediate C, there are two possible reaction path-
ing experiments, which indicate that triphenylphos- ways. In path a, ring expansion occurs to generate in-
phine plays the role as a reagent, not as a catalyst termediate E via transition state D. Intermediate E is
(Figure 1, for details, see the Supporting Informa- tautomerized to E’, which abstracts a proton from
tion).
water to produce intermediate F, followed by attack
Furthermore, the isotopic labeling experiments of hydroxide at the phosphonium ion before F collap-
were performed. As shown in Scheme 1, the control ses to Ph3P=O and intermediate G.[19] Finally, G un-
experiments were carried out by adding two equiv. of dergoes aromatization, leading to 2a. The nucleophi-
D2O (D content=99.96%) or H218O (18O content= licities of phosphines with alkyl groups are generally
98%) into the Ph3P/THF system, and the product was superior to those of phosphines with aryl groups,[20]
1
analyzed by H NMR spectroscopy and GC-MS (see meanwhile, the phosphines with alkyl groups are
the Supporting Information). We found that in the more basic than those phosphines with aryl groups.[21]
presence of H218O, 2a was formed with no 18O-label in The nucleofugicities of phosphines with alkyl groups
48% isolated yield along with a 29% yield of are presumably also superior to those of phosphines
18O=PPh3, in which the 18O content is ca. 60%. The with aryl groups.[22] Thus, in path b, from intermediate
result indicated that the oxygen atom of the triphenyl- C involving a phosphine with alkyl groups it is proba-
phosphine oxide was transferred from H218O. In the bly easier to release phosphine, generating a,b-unsa-
presence of D2O (2.0 equiv.), 3c-d was formed in 41% turated ketone I via transition state H, followed by
yield with D contents of 75% and 84%, respectively. attack of water at the carbonyl group to produce in-
3548
asc.wiley-vch.de
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 3545 – 3552

Phosphine-Promoted Cyclization of Dicyclopropenones
Figure 1. Reaction of 1a with PPh3 monitored by 31P NMR spectroscopy (400 MHz, CDCl3). A=PPh3, B=Ph3PO.
Scheme 1. Isotopic labeling experiments.
termediate J, which undergoes two-fold proton trans- SI-1 (for details, see the Supporting Information). In
fer and releases carbon dioxide via intermediate K to the case of PMe3, two possible reaction pathways
give 3a as the major product.
were investigated theoretically, and the calculated full
To understand clearly the mechanism of phosphine- reaction pathways are shown in Scheme SI-2 (for de-
promoted intramolecular cyclization of dicycloprope- tails, see the Supporting Information). As shown in
nones, we have theoretically investigated the reaction Scheme 3, the reaction starts from conjugate addition
pathways. All calculations have been performed at of PMe3 to cyclopropenone 1, affording zwitterionic
the mPW1K/6-31G(d) level with the Gaussian 09 pro- intermediate 4. The zwitterionic intermediate 4 under-
gram.[23] The DFT calculations support our proposed goes the carbon-carbon bond formation to afford in-
PPh3-promoted intramolecular cyclization of dicyclo- termediate 6 via transition state 5 with an energy bar-
propenones shown in Scheme 3 and the calculated full rier of 19.8 kcalmolÀ1. Subsequently, 6 undergoes an-
reaction pathway for this process is shown in Scheme other carbon-carbon bond formation to afford inter-
Adv. Synth. Catal. 2013, 355, 3545 – 3552
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3549

Jin-Ming Yang et al.
COMMUNICATIONS
Scheme 2. A plausible reaction mechanism.
mediate 8 via transition state 7 with an energy barrier and control experiments, and is also supported by
of 6.6 kcalmolÀ1. At this stage, the reaction may di- DFT calculations. Further applications of this novel
verge, and two distinct pathways lead to products 2 or phosphine-promoted cyclization and more detailed
3, respectively. In path a, passing through transition mechanistic investigations are underway in our labo-
state 9 with an energy barrier of 18.1 kcalmolÀ1, ratory.
a ring expansion occurs to give intermediate 11,
which continues to react with H2O, leading to product
2. In path b, passing through transition state 10 with
a lower energy barrier of 5.6 kcalmolÀ1, the PMe3 is
released to afford intermediate 12, which continues to
Experimental Section
react with H2O, leading to product 3. In this case, the
path b is kinetically favorable, and this calculation
result may account for the fact that the product 3 is
experimentally obtained as the main product (for the
General Procedure
Under an argon atmosphere, dry DCM (1.0 mL) containing
1.0 equiv. of H2O was added to a mixture of 1 (0.10 mmol)
and dppm (19.2 mg, 0.05 mmol). The reaction system was
stirred for 5 h at room temperature until 1 was completely
details, see the Supporting Information).
In summary, we have developed a novel phosphine-
consumed as shown by TLC monitoring. Then the solvent
promoted cyclization of dicyclopropenones system.
was removed under reduced pressure. The residue was puri-
18O-labeling experiments strongly supported the
fied by a silica gel flash column chromatography (eluent: pe-
troleum ether/EtOAc, 5/1) to give the product 2 as a color-
À
cleavage of the C P bond. The reaction mechanism
has been proposed on the basis of isotopic labeling less solid.
3550
asc.wiley-vch.de
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 3545 – 3552

Phosphine-Promoted Cyclization of Dicyclopropenones
Scheme 3. Theoretical investigations for the formation of 2a and 3a.
Breslow, L. J. Altman, A. Krebs, E. Mohacsi, I. Murata,
R. A. Peterson, J. Posner, J. Am. Chem. Soc. 1965, 87,
1326–1331.
Supporting Information
Spectroscopic data of the compounds shown in this article,
the detailed descriptions of experimental procedures and
the crystal structures of 2a and 3a are available in the Sup-
porting Information. CCDC 884906 (2a) and CCDC 902788
(3a) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.ca-
m.ac.uk/data_request/cif.
[2] D. N. Kursanov, M. E. Volpin, Y. D. Koreshkov, Izv.
Akad. Nauk SSSR, Otd. Khim. Nauk 1959, 560.
[3] a) T. Eicher, J. L. Weber, Top. Curr. Chem. 1975, 57, 1–
109; b) K. Komatsu, T. Kitagawa, Chem. Rev. 2003,
103, 1371–1428.
[4] K. T. Potts, J. S. Baum, Chem. Rev. 1974, 74, 189–213.
[5] Z. Yoshida, H. Konishi, in: Houben-Weyl – Methods oif
Organic Chemistry, 4th ed., Vol. El7d, (Ed.: A. de Mei-
jere), Thieme, Stuttgart, 1997, Chapter 5.
[6] R. Gleiter, M. Merger, Angew. Chem. 1997, 109, 2532–
2546; Angew. Chem. Int. Ed. Engl. 1997, 36, 2426–2439.
[7] B. Halton, M. G. Banwell, in: The Chemistry of the Cy-
clopropyl Group, (Ed.: Z. Rappoport), Wiley, New
York, 1987, p 1300.
[8] a) J. Ciabattoni, E. C. Nathan, J. Am. Chem. Soc. 1969,
91, 4766–4771; b) E. V. Dehmlow, R. Neuhaus, H. G.
Schell, Chem. Ber. 1988, 121, 569–571; c) S. S. Dehm-
low, E. V. Dehmlow, Z. Naturforsch. B. 1975, 30, 404–
408; d) E. V. Dehmlow, Leibigs Ann. 1969, 729, 64–68.
[9] D. H. Wadsworth, B. A. Donatelli, Synthesis 1981, 285–
286.
Acknowledgements
We thank the Shanghai Municipal Committee of Science and
Technology (11JC1402600), the National Basic Research Pro-
gram of China ACHTUNGTRENNUNG(973)-2010CB833302, and the National Natu-
ral Science Foundation of China (21072206, 21102166,
20472096, 20872162, 21372241, 21302203, 21361140350,
20672127, 21121062 and 20732008) for financial support.
References
[1] a) R. Breslow, R. R. Haynie, J. Mirra, J. Am. Chem.
Soc. 1959, 81, 247–248; b) R. Breslow, R. Peterson, J.
Am. Chem. Soc. 1960, 82, 4426–4427; c) R. Breslow, J.
Posner, A. Krebs, J. Am. Chem. Soc. 1963, 85, 234–234;
d) R. Breslow, T. Eicher, A. Krebs, R. A. Peterson, J.
Posner, J. Am. Chem. Soc. 1965, 87, 1320–1325; e) R.
[10] a) Y. Chiang, A. J. Kresge, V. V. Popik, J. Am. Chem.
Soc. 1995, 117, 9165–9171; b) Y. Chiang, A. J. Kresge,
S. W. Paine, V. V. Popik, J. Phys. Org. Chem. 1996, 9,
361–370; c) Y. Chiang, A. J. Kresge, V. V. Popik, J. Am.
Chem. Soc. 1999, 121, 5930–5932; d) A. Poloukhtine,
Adv. Synth. Catal. 2013, 355, 3545 – 3552
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3551

Jin-Ming Yang et al.
COMMUNICATIONS
V. V. Popik, J. Org. Chem. 2003, 68, 7833–7840; e) A. V.
Kuzmin, V. V. Popik, Chem. Commun. 2009, 5707–5709.
[11] a) B. D. Wagner, M. Z. Zgierski, J. Lusztyk, J. Am.
Chem. Soc. 1994, 116, 6433–6434; b) G. Quinkert, K.
Opitz, W. W. Wiersdorff, J. Weinlich, Tetrahedron Lett.
1963, 4, 1863–1868; c) O. L. Chapman, J. Gano, R. P.
West, M. Regitz, G. Maas, J. Am. Chem. Soc. 1981, 103,
7033–7036; d) A. Krebs, W. Cholcha, M. Mueller, T.
Eicher, H. Pielartzik, H. Schnoeckel, Tetrahedron Lett.
1984, 25, 5027–5030; e) S. Murata, T. Yamamoto, H.
Tomioka, J. Am. Chem. Soc. 1993, 115, 4013–4023;
f) K. Sung, D.-C. Fang, D. Glenn, T. T. Tidwell, J.
Chem. Soc. Perkin Trans. 2 1998, 2073–2080.
[12] a) I. Agranat, A. Barak, M. R. Pick, J. Org. Chem.
1973, 38, 3064–3065; b) T. Eicher, V. Schmieder, Syn-
thesis 1989, 372–378; c) C. H. Weidner, D. H. Wads-
worth, C. S. Knop, A. I. Oyefesso, B. L. Hafer, R. J.
Hartman, R. C. Mehlenbacher, S. C. Hogan, J. Org.
Chem. 1994, 59, 4319–4322.
[13] a) D. L. Boger, C. E. Brotherton, J. Am. Chem. Soc.
1986, 108, 6695–6713; b) H. Isobe, S. Sato, T. Tanaka,
T. H. Okuyama, E. Nakamura, Org. Lett. 2004, 6, 3569–
3571; c) P. A. Wender, T. J. Paxton, T. J. Williams, J.
Am. Chem. Soc. 2006, 128, 14814–14815; d) P. A.
OꢀGorman, T. Chen, H. E. Cross, S. Naeem, A. Pitard,
M. I. Qamar, K. Hemming, Tetrahedron Lett. 2008, 49,
6316–6319; e) M. Alajarꢃn, C. Lꢁpez-Leonardo, ꢄ. ꢄl-
varez-Garcꢃa, P. Llamas-Lorente, P. Sꢅnchez-Andrada,
J. Bernꢅ, A. Pastor, D. Bautista, P. G. Jones, Chem. Eur.
J. 2010, 16, 3728–3735.
[14] a) A. Hamada, T. Takizawa, Tetrahedron Lett. 1972, 13,
1849–1850; b) A. Kascheres, A. C. Joussef, H. C.
Duarte, Tetrahedron Lett. 1983, 24, 1837–1840.
[15] a) B. D. Kelly, T. H. Lambert, J. Am. Chem. Soc. 2009,
131, 13930–13931; b) D. J. Hardee, L. Kovalchuke, T. H.
Lambert, J. Am. Chem. Soc. 2010, 132, 5002–5003;
c) C. M. Vanos, T. H. Lambert, Chemical Science 2010,
1, 705–708; d) B. D. Kelly, T. H. Lambert, Org. Lett.
2011, 13, 740–743; e) C. M. Vanos, T. H. Lambert,
Angew. Chem. 2011, 123, 12430–12434; Angew. Chem.
Int. Ed. 2011, 50, 12222–12226; f) E. D. Nacsa, T. H.
Lambert, Org. Lett. 2013, 15, 38–41.
[16] See the Supporting Information for details.
[17] a) W. Zhang, M. Shi, Chem. Commun. 2006, 1218–
1220; b) M. Shi, X.-G. Liu, Y.-W. Guo, W. Zhang, Tet-
rahedron 2007, 63, 12731–12734; c) S.-H. Cao, X.-C.
Zhang, Y. Wei, M. Shi, Eur. J. Org. Chem. 2011, 2668–
2672; d) Y. Wei, X.-G. Liu, M. Shi, Eur. J. Org. Chem.
2012, 2386–2393.
[18] See the Supporting Information for details.
[19] I. Erden, C. Gꢆrtner, M. S. Azimi, Org. Lett. 2009, 11,
3986–3989.
[20] B. Kempf, H. Mayr, Chem. Eur. J. 2005, 11, 917–927.
[21] K. Haav, J. Samme, A. Kꢇtt, I. Leito, Eur. J. Org.
Chem. 2012, 2167–2172.
[22] For Mayrꢀs findings on nitrogen-containing organocata-
lysts, see: M. Baidya, S. Kobayashi, F. Brotzel, U.
Schmidhammer, E. Riedle, H. Mayr, Angew. Chem.
2007, 119, 6288–6292; Angew. Chem. Int. Ed. 2007, 46,
6176–6179.
[23] M. J. Frisch, et al. Gaussian 09, revision A.01, Gaussian,
Inc., Wallingford, CT, 2009.
3552
asc.wiley-vch.de
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 3545 – 3552